(1) Three phosphoinositide-specific phospholipase C isozymes (PLC- I, II, and III) have been purified from bovine brain. The phospholipase C-related cDNA clones corresponding to PLC-I, II, and II were isolated from rat brain lambdal gt11 expression cDNA library using specific monoclonal antibodies and sequenced. Each of them encodes a distinct polypeptide with a calculated molecular mass of 138,225 (PLC-I), 148,431 (PLC-II), and 85,840 (PLC-III). Comparison of the sequence of these three isozymes revealed a low overall sequence homology. Nevertheless, a significant amino acid sequence similarity among the three enzymes was found in two regions, one region of about 150 amino acids and the other of about 120 amino acids. The two conserved domains were separated by a variable region. The variable region sequence of PLC-II is relatively long and contains regions homologous to the noncatalytic domain of the nonreceptor protein tyrosine kinases, indicating that PLC-II and cytoplasmic tyrosine kinases might be regulated by a common cellular component. The variable regions of PLC-I and III were short and appeared to be unrelated to these tyrosine kinases. Thus, the modulation of all three enzymes might be quite different. (2) Two forms of inositol 1,4,5-trisphosphate 3-kinase from bovine brain IPK-1 were partially purified and characterized. IPK-1 was activated about 6-fold by calmodulin in the presence of calcium, whereas IPK-2 is nearly insensitive to CaM.